The invention relates to a switching network for a switching system in which cells are transmitted in accordance with a time division multiplex method, particularly asynchronous time division multiplex methods, and the through-connection of the cells in the switching facility is effected by means of the routing information contained in the header of the cells and which is configured in the manner of a space division multiplex switching network as bus system having offering trunks and serving trunks. In this arrangement, the offering trunks are preferably arranged row by row and the serving trunks column by column.
The proportion of data traffic to be transmitted is continuously increasing in communication networks. To handle the data traffic, separate switching systems have been built up for the data and the telephone traffic. Since the data rate of the data signals to be transmitted can assume a plurality of different values, it is difficult to combine data and telephone switching in one network node.
Within the context of the development of future integrated broadband networks, the question is being discussed which method is to be used for transmitting the digital or analog information--partially with a high bandwidth requirement--in digital networks. Packet switching has long been known, in which the (digital) information stream is divided into individual packets. Each packet contains additional information, among other things information on the destination (address), in addition to the user information. As a result, messages can be directed to a party involved in the connection on the basis of their packet addresses, which only requires transmission capacity when a message is actually to be transmitted.
Furthermore, circuit switching has long been known, which requires transmission capacity for the entire duration of the cell, independently of whether messages are being transmitted or not. Particularly in the case of the telephone network, two-way transmission paths are provided although generally only one of the two telephone subscribers is speaking.
The use of packet switching in data traffic has previously led to throughputs of the order of magnitude of 1,000 packets per second and more. 1,000 times this amount is already expected today for future services using the method of packet switching, for example in the case of video communication. To achieve such throughputs, the time-wasting handling of the protocols is separated from the actual through-connection process in fast packet switching and the packets are distributed to the transmission paths in accordance with their destination addresses. To accelerate the through-connection process in the switching networks, greatly simplified protocols are used which can be very rapidly interpreted. At the input of the switching node, the incoming data packets are processed in a non-centralized manner by being provided with an internal address information according to their destination, connected through the central switching network and then forwarded at the output without the internal address information.
"Draft Recommendation I" issued by the CCITT Study Commission XVIII describes the "Asynchronous Transfer Mode". In the synchronous transfer mode, a particular transmission channel is only identified by its separation in time from a synchronization word. In the asynchronous transfer mode, the respective channel is identified by the addresses (headers) of its packets (cells) and, in particular, the start of the cells can be recognized, so that the headers can be interpreted. For this purpose, flags or a lower-level synchronization structure is used which presupposes a constant cell length. An example of this is "asynchronous time division", in which synchronization words are inserted again and again as "dummy cells" into the bit stream.
Initially, hybrid configurations with a combination of "synchronous transfer mode" and "asynchronous transfer mode" should attain practical significance since the network operator would like to continue to use the already existing networks configured in accordance with the synchronous transfer mode as long as possible due to the high investment costs.
In the European patent application having the application number 0,183,592, a broadband switching system was proposed in which the message is split into cells which are transmitted via broadband transmission paths in accordance with an asynchronous time division multiplex method. The cells can exhibit equal or different lengths. The cells consist of user and of address information, the address information being accommodated in a so-called header. The number of bits of a cell is designated as its length, values of between 120 and 256 bits for the user information and 32 or 16 bits for the header being provided for this purpose in the standardization proposals. The time intervals in which cells are transmitted are called frames. In this connection, a frame can contain a valid cell or be empty. Between two subscribers of the broadband switching system, a "virtual connection" exists which is maintained by the fact that the cells transmitted by the subscriber devices are provided with unambiguous header identifications which enable the switching node to forward the cells correctly. The cells arriving from an incoming trunk at the switching node are transferred to an outgoing trunk with conversion of the header. Since two or more cells can arrive for the same output trunk during a frame, so-called queuing buffers must be provided in the switching node. In the queuing buffer, one or more of these cells are temporarily stored until a free frame is available for these.
Having regard to the queuing buffer arrangement, the switching nodes can be centrally buffered systems (as known, for example, from the European patent application having the application number 0 183 592) or non-centrally buffered systems. In centrally buffered systems, there is only one buffer in which each incoming trunk deposits its incoming cells and from which each outgoing trunk reads out again the cells intended for it. Systems having noncentralized buffering are distinguished with respect to whether cells are buffered exclusively on the input side (an embodiment of this is described, for example, in German Patent Application P No. 3,714,385.9) or whether the buffers are exclusively arranged upstream of the outgoing trunks (compare, for example, IEEE, B 10.2.1, 1987, "The Knockout Switch: A Simple, Modular Architecture for HighPerformance Packet Switching by J. S. Yeh et al) or whether these are systems with input and output buffering. This is also called switching network buffering if a buffer is allocated to each switching point of a switching matrix.
As is shown by the details above, the design of the switching network for a switching node of a broadband switching system requires tedious investigations and special considerations for taking into account the manifold relationships and their mutual interactions. In addition, it must be taken into account that the circuit technology for the switching networks considered is at the limits of present semiconductor switching times. In particular, performance bottlenecks can occur if several cells are simultaneously sent through the switching network of a switching system. The cells can influence each other if fewer links than necessary paths are available in the switching network. In this connection, two types of interaction are of particular significance: contention and congestion.
Two cells (or the circuits sending them out) are in contention with one another if the same circuit parts are to be used for the transmission. As a rule, one of the cells is then prioritized compared with the other one and the other cell must either wait or is lost.
In congestion, a cell V has to wait for a cell U to be processed, but at the same time cell U is not processed because there is contention between cell U and a third cell W. A normal queuing situation in a queuing buffer is thus not a contention situation as long as the first cell in the buffer is transmitted in each frame cycle. Congestion occurs only if this cell enters into contention and is not processed but as a result, at the same time, no other cell in this queuing buffer can be transferred either.
The concept of congestion is of importance for the evaluation of the performance of such systems. This is because if a queuing buffer which is not empty cannot be processed during one frame period, it can be imagined that, instead, an additional "virtual" cell is processed. The sum of virtual and real load then results in the total load carried by the system. In some switching networks, the virtual load can become almost as great as the real load. This mainly applies to switching networks which use input buffering.
In German Patent Specification No. 3,743,685.6, a switching network having input buffering has been proposed which is configured of a bus system having offering trunks arranged row by row and serving trunks arranged column by column in the manner of a space division multiplex switching network. For evaluating the information items contained in the header, each switching point is provided with its own evaluating logic. At the input of each switching point, an input buffer is provided which receives and stores the cells supplied via the offering trunk until they reach one of the serving trunks. The number of input buffers corresponds to the number of serving trunks. The frames received by the subscriber or by the preceding switching node are located right-justified in the input buffer. A comparator allocated to each switching point is used for comparing the address, stored in a store, of the serving trunks arranged column by column with the routing information contained in the header and stored in a shift register. For this purpose, the shift register is connected, on the one hand, to the comparator and, on the other hand to the switching point on the output side of the buffer store. To each of the serving trunks a decision circuit is allocated which determines the order of offering trunks to be connected through in the case of identical routing information items. The order is determined by the spatial arrangement of the offering trunks, in which arrangement each decision circuit cyclically polls all comparators allocated to it.
In this switching facility, the buffer stores allocated to a serving trunk contain data which are not intended for this serving trunk. With a number of n serving trunks, an average of (n-1/n).times.100% of all buffer spaces are unnecessarily occupied with data in this manner. These buffer spaces, however, must be available, which is why considerably more buffer space must be provided than would actually be required.